1. Field of the Invention
This invention relates generally to electronically controlled transmission/engine drive train systems and more specifically to those having a manual/automatic transmission wherein the transmission includes manually selectable gear ratio operating states and automatically or electronically selectable gear ratio operating states operable in conjunction with electronic cruise control systems.
The present invention relates to control systems and methods of operation for manual/automatic heavy duty engine/transmission systems wherein gear selection and gear shifting timing decisions are made and/or executed based upon measured and calculated parameters such as gear engagement condition of the transmission, vehicle or transmission output shaft speed, transmission input shaft speed, engine speed, throttle position, and the like. Until recently, transmissions, particularly for heavy duty applications, have historically been the manual shift type requiring actuation of a clutch and gear shift lever by the driver in order to change final drive gear ratios. Final drive gear ratio selection transmission via a shift linkage, for mechanically disengaging and engaging various available final drive gear ratios of the transmission. Due to the large number of gear ratios associated with compound transmissions or transmissions having a splitter type auxiliary section, a heavy burden is borne by the driver to manually shift throughout the entire set of available final drive gear ratios or operation states of the transmission. Even after the vehicle has reached highway cruising speeds, road grade variations demand that the driver shift between the highest two gear ratios to maintain vehicle speed relatively constant. More recently, successful devices that combine manual and automatic shifting operation under certain conditions have been developed such as the invention disclosed in U.S. Pat. No. 4,722,248, the disclosure of which is incorporated by reference.
When an electronic shift control system is coupled to a manual/automatic transmission, a lessening of operator fatigue and improved fuel economy result. A device which combines manual shifting of lower vehicle speed final drive gear ratios with automatic shifting between sequentially related higher vehicle speed final drive gear ratios, and more particularly between the "top two" available gear ratios of the manual/automatic transmission will serve to reduce driver fatigue. By providing for automatic gear selection of the two highest gear ratio operation states of the transmission, or the "top two" gears, an automatic shift schedule can be developed for increasing fuel economy. Additionally, an automatic gear shift device can be used in conjunction with cruise control electronics systems compatible with heavy duty truck applications.
Considerations related to the operation of a cruise control system for a heavy duty truck hauling a very heavy payload give rise to issues relating to when a gear shift should occur between the top two gears of the manual/automatic transmission. If the shift point is strictly mile per hour based, maximum fuel efficiency is compromised due to excessively high or low engine RPM. In fact, the system may never shift gears at all if the cruise speed is set below the mile per hour shift point for the manual/automatic transmission cruise control system. On the other hand, if the shift point is strictly a function of RPM, fuel efficiency may improve, but driveability of the vehicle is compromised due to the fact that the acceleration capability of the vehicle is fixed through vehicle control electronics. Here again, the vehicle may never shift if cruise speed, i.e. the engine RPM, is set below the RPM shift point. In addition, the load borne by the engine of the vehicle in response to climbing an incline at full payload provides a factor or element in the decision-making process for determining what gear ratio is appropriate under these conditions. Shift timing is lost and driveability is compromised if the loading of the vehicle engine versus peak torque and power output levels are not considered in determining at what speed automatic gear shifts should occur.
Thus, a control system for controlling the shift points in a variable fashion for a cruise control system coupled to a heavy duty truck transmission having at least two automatically selectable gear ratio operation states is needed for maximizing efficiency and improving driveability of the vehicle.